This invention relates to the direct attachment of flip chips to printed circuit boards (PCBs).
One problem with direct attachment of a flip chip to a PCB is the difference in coefficient of thermal expansion between the chip (made of silicon) and the PCB (made of organic material such as reinforced epoxy resin). This produces stresses on the solder joint, which can result in failure of the joint through fatigue.
One known way of alleviating this problem is to fill the gap between the chip and the PCB with epoxy material. The material is applied in liquid form and drawn into the gap by capillary action. The material takes stresses away from the solder, and has been found to increase the fatigue life of the joint by a factor of ten. However, a problem with this is that the larger the chip the longer the time needed to draw the epoxy material into the gap. Possibly several passes are required to fill the gap completely, which causes problems with manufacturing. Another problem is that, after the gap is filled, removal and replacement of a defective chip is very difficult.
The object of the present invention is to provide an alternative technique for flip chip attachment which avoids these problems.